Epoxy functional UV and thermally curable materials are ubiquitous in the fields of adhesives, coatings, films and composites. The benefits of utilizing epoxy-based materials include generally good adhesion, widely variable curing mechanisms and curing rates, fairly cheap and readily available raw materials and good chemical resistance. The widespread use and longevity of epoxy technology is testament to its utility even in the face of more recently developed chemistries such as cyanate esters and maleimide resins, to name a few. In spite of the general acceptance of typical epoxy materials, several deficiencies are recognized within the industries which utilize thermosetting and UV curable materials. Common epoxy resins, chemically described hereafter, typically cure to relatively rigid, high Tg materials. Also, the upper use temperature of epoxy-based materials is generally in the region of 150° C. to 180° C., somewhat lower than that required for many demanding application areas. Lastly, the moisture uptake of most epoxy materials under high humidity conditions is on the order of several weight percent. This level of moisture absorption is undesirable for many applications, particularly in the areas of electronics adhesives and coatings. weight percent. This level of moisture absorption is undesirable for many applications, particularly in the areas of electronics adhesives and coatings.
The most common epoxy resins are aromatic molecules such as bisphenol A diglycidyl ether (DGEBPA) or epoxidized novolak resins (such as the EPON® series of resins sold by Shell Chemical). These resins, derived from the reaction of epichlorohydrin with alcohols (or an equivalent synthetic process), are most commonly utilized for thermally curing applications. For UV curable systems, cycloaliphatic type epoxy systems (such as ERL 4221 or ERL 6128 sold by Union Carbide) are more commonly used due to their rapid cationic curing kinetics. Rubberized epoxies, commonly derived from chain extension of amino- or carboxyl-terminal rubbers with bis(epoxides), are typical film forming epoxy-functional materials. All of these systems suffer from one or more of the aforementioned deficiencies of epoxy-based systems. The rigidity of most commercial cured cycloaliphatic epoxy materials is particularly notable.
One approach to improving the flexibility, thermal stability and moisture resistance of classic epoxy materials is the incorporation of siloxane-based resins into the cured epoxy matrix. Various approaches have been taken toward this end, including chain extension of bis(epoxides) with carbinol-terminal siloxanes and the synthesis of a variety of “epoxysiloxanes” via the hydrosilation of unsaturated epoxides onto SiH-functional siloxane materials. With regard to the latter class of materials, attempts have been made to fully consume as much of the SiH functionality as possible during these syntheses, as it has been correctly noted that the presence of SiH functionality, epoxide functionality and residual transition metal catalyst (especially platinum) leads to variably unstable products. It is well known to those practiced in the art that complete consumption of the silicon-hydride functionality on many silicone backbones is a challenging synthetic goal.
The use of rhodium based catalysts has been shown to reduce the tendency for epoxide functionality to polymerize in the presence of SiH groups during these hydrosilation reactions. Techniques involving the monohydrosilation of certain classes of disilanes and disiloxanes have been utilized to yield SiH-functionalized molecules and intermediates. Several literature citations note the possibility of synthesizing a material with both SiH and epoxy functionality. The limited examples involving the use of these intermediates do not produce products with highly controlled molecular geometries and/or epoxy contents. Epoxy-endcapped linear copolymers of silicon hydride-terminal poly(dimethylsiloxane)s and difunctional polyethers (typically allyl-terminal poly(proylene glycol) have also been described. The resulting linear copolymers exhibit improved compatibility with organic materials. Such linear copolymers are limited by their necessarily bis-functionality (at most two epoxy groups per linear polymer), and have not been extended to incorporate silane inorganic repeat units or organic dienes beyond those derived from poly(ethers). This significantly reduces the utility of these polymeric materials in applications which demand reasonably high levels of crosslink density. The molecular architecture of these linear copolymers is not well defined, in that such materials exhibit the statistical distribution of molecular weights typical of “one step” polymerizations. The general effects of molecular weight distribution on material and viscoelastic properties are well known.
The synthesis and use of either SiH-terminal or olefin-terminal diene-siloxane copolymers (precursors to the epoxy-functional materials discussed above) has also been documented, but synthetic strategies have not been developed to allow for extension to radial structures as discussed herein.
In general, resins known in the prior art containing both epoxide and siloxane functionality exhibit poor compatibility with common, industrially useful, epoxide resins such as epoxy novolaks, DGEBPA and representative cycloaliphatic epoxides such as ERL-4221 and ERL 6128 described above. This poor “organic compatibility” of “epoxysiloxanes” known in the prior art is well known. Most often, macroscopic phase separation quickly occurs when blends with hydrocarbon resins are attempted. Although the functionalization of siloxane materials with alkyleneoxy sidechains is known to enhance compatibility in some organic materials, for many applications (such as electronics adhesives and coatings) the increased hydrophilicity of the resulting siloxane materials is problematic.
It is therefore one intention of the current invention to provide industrially feasible syntheses of hydrophobic epoxysiloxanes with good compatibility in common hydrocarbon-based epoxy resins. It is further our intention to present the synthesis of novel linear and “radial” geometry epoxy-functional siloxane or silane/hydrocarbon copolymers with 1) highly controllable molecular geometry (polydispersities of approximately one), 2) tailorable silicon:hydrocarbon ratios, and 3) variable levels of epoxy functionality (typically greater than two). Finally, the inventive materials of this application exhibit several desirable features not found in the materials of prior art such as: 1) improved hydrocarbon compatibility relative to most commercial epoxysiloxane resins, 2) improved hydrophobicity relative to hydrocarbon-based epoxies, 3) improved thermal stability relative to hydrocarbon-based epoxies, 4) high UV reactivity relative to many commercial epoxies, and 5) improved material properties relative to typical cycloaliphatic epoxies used for UV cure applications.
Additionally, it is recognized that the intermediate olefin terminal and SiH terminal radial copolymers of the current invention are also novel and useful. For example, alkenyl-terminal resins may be used as reactive intermediates alone or in combination with other materials. Similarly, SiH-terminal materials may be used as reactive crosslinkers for hydrosilation cure compositions.